1. Field of the Invention
The present invention relates to an EL (electro-luminescence) display device fabricated by forming a semiconductor device (a device utilizing a semiconductor thin film; typically a thin film transistor) onto a substrate. The present invention further relates to an electrical equipment including such an EL display device as a display section.
2. Description of the Related Art
Recently, a technique for forming a thin film transistor (hereinafter referred to as TFT) onto a substrate has significantly advanced, and its application to an active-matrix display device has been developed. In particular, the TFT employing a polysilicon film therein has a field effect mobility higher than that of the conventional TFT employing an amorphous silicon film, and therefore, can operate at higher speed. Thus, a control function for pixels, that is conventionally performed by an external driver circuit provided at the outside of the substrate, can be performed by a driver circuit that is provided on the same substrate as the pixels.
The active-matrix display device as mentioned above can provide various advantages such as reduction in the manufacturing cost, downsizing of the display device, improvement of the yield, reduction in the throughput or the like, when various circuits and/or devices are fabricated on one and the same substrate. Thus, this kind of active-matrix display device has drawn much attention.
In an active-matrix EL display device, a switching device employing a TFT (hereinafter referred to as switching TFT) is provided at each pixel, and each of the respective switching TFTs allows a corresponding drive device for controlling current (hereinafter referred to as current-controlling TFT) to drive, thereby causing an EL layer (more strictly speaking, a light emitting layer) to emit light. An exemplary EL display device is described, for example, in Japanese Patent Application Laid-Open No. Hei. 10-189252.
The EL display device includes a device section composed of a cathode, an EL layer, and an anode (hereinafter, the device composed of these portions is referred to as EL device). When a film resistance of the anode in the device section increases, the in-plane distribution of electrical potentials in the anode becomes non-uniform due to the voltage drop, thereby resulting in disadvantages such as deviations in the light intensity of the EL device.
Accordingly, an object of the present invention is to provide an EL display device having the structure capable of lowering a film resistance of an anode in an EL device or exhibiting any corresponding advantages. Furthermore, another objective of the present invention is to provide electrical equipment having a display section which operates stably by employing such an EL display device as the display section.
The present invention will be described below with reference to FIG. 1. In FIG. 1, reference numeral 101 denotes a substrate having an insulating surface. As the substrate 101, for example, an insulating substrate such as a quartz substrate can be used. Alternatively, various kinds of substrate, such as a glass substrate, a ceramic substrate, a crystallized glass substrate, a metal substrate, or a plastic substrate, can be used by providing an insulating film on a surface thereof.
On the substrate 101, pixels 102 are formed. Although only three of the pixels are illustrated in FIG. 1, a higher number of pixels are actually arranged in matrix. Further, only one of the three pixels will be described below, but the other pixels have the same configuration as the explained one.
In each of the pixels 102, two TFTs are formed; one of them is a switching TFT 103, and the other is a current-controlling TFT 104. A drain of the switching TFT 103 is electrically connected to a gate of the current-controlling TFT 104. Furthermore, a drain of the current-controlling TFT 104 is electrically connected to a pixel electrode 105 (which, in this case, also functions as a cathode of an EL device). The pixel 102 is thus formed.
Various wirings of the TFT as well as the pixel electrode can be formed of a metal having a low resistivity. For example, an aluminum alloy may be used herein for this purpose.
Following the fabrication of the pixel electrode 105, an insulating compound (referred to as alkaline compound hereinafter) 106 containing an alkaline metal or an alkaline-earth metal is formed. It should be noted that the outline of the alkaline compound 106 is indicated by a dotted line in FIG. 1 because the compound 106 has a thickness which is as thin as several nanometers, and it is not clear whether the compound 106 is formed as a layer or in an island-shape.
As the above-mentioned alkaline compound 106, lithium fluoride (LiF), lithium oxide (Li2O), barium fluoride (BaF2), barium oxide (BaO), calcium fluoride (CaF2), calcium oxide (CaO), strontium oxide (SrO), or cesium oxide (Cs2O) can be used. Since these are insulating materials, electrical short-circuiting between the pixel electrodes does not occur even when the compound 106 is formed as a layer.
It is of course possible to use a known conductive material such as a MgAg electrode as the cathode. However, in such a case, in order to avoid electrical short-circuiting between the pixel electrodes, the cathode itself has to be selectively formed or patterned into a certain shape.
Once the alkaline compound 106 is formed, an EL layer 107 (an electro-luminescence layer) is formed over the compound 106. Any known material and/or structure can be employed for the EL layer 107. More specifically, with respect to the structure of the EL layer, only a light emitting layer for providing sites for the carrier recombination may be included in the EL layer. Alternatively, if necessary, an electron injection layer, an electron transport layer, a hole transport layer, an electron blocking layer, a hole device layer, or a hole injection layer may be further layered to form the EL layer. In the present application, all of those layers intended to realize injection, transport or recombination of carriers are collectively referred to as the EL layer.
As an organic material to be used as the EL layer 107, either a low-molecular type organic material or a polymer type (high-molecular type) organic material can be used. However, it is desirable to use a polymer type organic material which can be formed by a film-formation method that can be easily performed, such as a spin coating method, a printing method, or the like.
The structure illustrated in FIG. 1 is an example of the monochrome color light-emitting type in which an EL layer for emitting a monochrome color light, such as a red color, a blue color, a green color, a white color, a yellow color, an orange color, a purple color or the like, is used for displaying a monotone image. The EL layer for emitting any monochrome color light as mentioned above may be formed of known materials.
Over the EL layer 107, a transparent conductive film is formed as an anode 108. As the transparent conductive film, a compound of indium oxide and tin oxide (referred to as ITO), a compound of indium oxide and zinc oxide, tin oxide, or zinc oxide (ZnO) can be used.
In the present application, a film resistance of the whole anode obtained by calculating an average of a film resistance for a region where a metal film 109 and the anode 108 are layered and a film resistance for only the anode (in other words, a film resistance of the whole portion electrically connected to the anode) will be referred to as the average film resistance of the anode. By providing the metal film 109 over the anode, the average film resistance in the anode can be decreased. Furthermore, the metal film 109 also functions as a light shielding film.
As a deposition technique for the metal film 109, a vapor deposition method is desirable in view of any possible damage to the anode during the deposition process.
In addition, upon the provision of the metal film 109, it is preferable to provide the metal film 109 so that gaps 111 between the adjacent pixel electrodes are concealed thereby when viewed from the viewing direction of a viewer (i.e., from the direction of the normal to a counter electrode). This is because of the fact that those gaps are non-light emitting regions, as well as the fact that the electrical field distribution becomes complicated in the vicinity of end portions of the pixel electrodes so that light emission at a desired light intensity or a desired chromaticity cannot be realized there.
After the metal film 109 is formed as mentioned above, an insulating film as a second passivation film 112 is provided. As the passivation film 112, a silicon nitride film or a silicon oxynitride film (represented as SiOxNy) is preferably used. Although it is possible to use a silicon oxide film as the passivation film 112, it is preferable to use an insulating film containing oxygen as little as possible.
The substrate fabricated up to this stage is referred to as an active-matrix substrate in the present application. More specifically, the substrate on which TFTs, pixel electrodes respectively electrically connected to the TFTs, as well as EL devices each composed of an EL layer, an anode, and a metal film and utilizing the corresponding pixel electrode as a cathode are formed is referred to as the active-matrix substrate.
Furthermore, a counter substrate 110 is attached to the active-matrix substrate so that the EL devices are interposed and sealed therebetween. Although not illustrated herein, the counter substrate 110 is adhered to the active-matrix substrate by means of a sealing agent, so that a space designated with reference numeral 113 becomes a closed space.
As the counter substrate 110, it is necessary to use a transparent substrate so as not to prevent light from passing therethrough. For example, a glass substrate, a quartz substrate, or a plastic substrate is preferably used.
The closed space 113 may be filled with inert gas (noble gas or nitrogen gas), or with inert liquid. Alternatively, the closed space 113 may be filled with a transparent additive agent or resin so as to adhere the whole surface of the substrate. Moreover, it is preferable to dispose a drying agent such as barium oxide or the like in the closed space 113. Since the EL layer 107 is very vulnerable to water, it is desirable to prevent water from entering the closed space 113 as much as possible.
In the EL display device having the above-described configuration in accordance with the present invention, light emitted from the EL device passes through the counter substrate to be emitted to reach the viewer""s eyes. Accordingly, the viewer can recognize an image through the counter substrate side. In this situation, one of the features of the EL display device in accordance with the present invention is that the metal film 109 having a low electrical resistivity is disposed on the anode 108 included in the EL device so that the gaps 111 between the adjacent pixel electrodes 105 are concealed by the metal film 109. This results in a decreased average film resistance of the anode in the EL device as well as prevention of light leakage from the gaps 111 between the pixel electrodes 105. Thus, an image can be displayed with clear contours between the pixels.
Thus, in accordance with implementing the present invention, an EL display device capable of having a reduced average film resistance of an anode in the EL device section as well as displaying an image with clear contours between the pixel electrodes can be provided. Furthermore, electrical equipment employing such an EL display device as a display section can be also provided.